Polycrystalline mullite fiber composite products vacuum forming block on full hydrogen bell furnace

1. Preface
The bell furnace annealing of cold rolled strip is one of the key processes in the production of cold rolled sheet. At present, the all-hydrogen bell furnace is the most advanced type of annealing furnace. Anshan Iron and Steel Cold Rolling Sheet Factory put 76 all-hydrogen bell furnaces into operation in 1991 and 1995 respectively. Among them, 36 all-hydrogen bell furnaces started ignition production in May 1995 have been improved and improved in many aspects through the suggestions of Chinese and foreign technical experts and users. The most important improvement is to replace the lightweight refractory brick hearth lining with polycrystalline mullite fiber composite hearth lining.

In 1988, Anshan Iron and Steel Cold Rolling Sheet Factory began to introduce the project of full hydrogen bell furnace. During the technical exchange and negotiation with LOI Company of Germany and EBNER Company of Austria, the foreign experts of the two companies all proposed that the furnace lining should be made of high-grade refractory materials. Since there was no domestic manufacturer of refractory materials such as Domo mullite fiber composite products at that time, if, in the project of four full hydrogen bell furnace lining imported from EBNER Company of Austria, foreign experts insisted on the need to introduce both lining and refractory materials, which cost a lot of foreign exchange. The lining of 36 all-hydrogen bell furnaces imported from German LOI company had to use light refractory bricks.

Anshan Iron and Steel Cold Rolling Sheet Factory signed a contract with German LOI Company in 1933 for the second time to introduce 36 full-hydrogen bell-type furnaces. For the first time, polycrystalline mullite fiber composite products were used instead of imported refractory materials. This kind of polycrystalline mullite fiber composite products have achieved considerable economic benefits through one year's production and use.

2. Furnace lining structure
REQUIREMENTS OF REFRACTORY MATERIAL FOR Lining OF 2.1 BLOVING FURNACE
The characteristics of bell furnace production are characterized by short annealing cycles and are intermittent. When the furnace charge is 70t, the annealing cycle of the heating cover is about 20 hours. After the heating and heat preservation of the heating cover on one furnace table, the heating cover should be lifted and unbuckled to another furnace table for heating and annealing. During the lifting process of about 5 minutes each time, the temperature of the heating cover lining inevitably decreases from about 800oC to about 500oC, in this way, the sudden change of the lining temperature of the heating cover every 20 hours requires the lining refractory material to have good quenching and rapid heating performance, as well as low specific heat and low linear expansion performance. The repeated frequent lifting of the heating cover requires the lining material to be low density and have certain seismic performance. Polycrystalline mullite fiber composite products vacuum forming block (PMF-300), not only low thermal conductivity, but also after sintering and drying, fully meet the requirements of the above-mentioned refractory furnace.

2.2 lining structure
The upper part of the lining of the heating cover of the hood furnace is aluminum silicate fiber. Since the lower part is the burner area, the flame hot air flow has a certain scouring effect on the lining, so light refractory bricks or polycrystalline mullite fiber composite products with a thickness of 30mm must be used between the vacuum blocks.
Generally, the temperature of the combustion space of the hood furnace is about 900oC. Since the heating cover is externally equipped with electrical components, computer components and burner controllers, the temperature of the outer surface of the heating cover is required to not exceed 50oC.

According to Fourier's law: q = λΔt/s, through trial calculation and checking calculation, the thickness of the lining made of lightweight refractory bricks is 345mm, and the thickness of the lining made of vacuum forming blocks made of polycrystalline mullite fiber composite products is 220mm.

Each heating hood has 12 burners, and the lining structure around the burner is relatively complicated. The vacuum forming block of polycrystalline mullite fiber composite products that is easy to process is used as the lining to prevent the gas energy of the burner and the combustion-supporting air mixed flow sheet from deforming, which can completely meet the design requirements.

3. Use effect and economic benefit
Comparison of 3.1 use effect
Lightweight refractory bricks are used as the lining of the burner area of the heating cover of the hood furnace. Cracks begin to appear in the lining about half a year after use. With the frequent lifting of the heating cover, the cracks become more serious and the outer surface temperature of the heating cover rises, which not only increases the heat loss, but also causes damage to the electrical components, computer components and burner controller on the heating cover, affecting the heating capacity of the heating cover and the normal production. Generally, after the new heating cover is used for half a year, the lining must be repaired. After one year, the repair is more frequent, and the life of the furnace lining cannot meet the production requirements.

The vacuum forming block of polycrystalline mullite fiber composite products is used as the lining of the heating hood burner area. After one year's use, the disadvantage of using light refractory bricks as the lining of the heating hood burner area of the hood furnace has been completely overcome. No cracks have been found. The temperature of the outer surface of the heating hood maintains the designed temperature. The production is normal. The mixed flow sheet is not deformed and burns normally.

3.2 economic benefits
3.2.1 Reduce furnace repair time and improve production capacity
Anshan Iron and Steel Cold Rolling Sheet Factory introduced 36 all-hydrogen bell furnaces and 19 heating hoods in the later period. One repair of the heating hoods takes 5 days to stop production. The production capacity of the heating hoods is 3.5 tons per hour. The production capacity of the bell furnace can be increased by 7980 tons per year by replacing the lightweight refractory bricks with genuine molded blocks of polycrystalline mullite fiber composite products. According to production statistics, reducing the number of replacement or repair burners can increase the production capacity by 6384 tons, and the two items can produce 14364 tons more cold-rolled plates.

3.2.2 Energy saving
After calculation, using the vacuum forming block of polycrystalline mullite fiber composite products as the lining of the heating hood burner area instead of light refractory bricks as the lining of the heating hood burner area instead of light refractory bricks as the lining of the heating hood burner area of the hood furnace can save energy every year due to the reduction of heat loss and the reduction of the temperature of the outer surface of the furnace body:
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3.3 reduce the consumption of imported spare parts
At present, the computer components spare parts and burner controller spare parts of the all-hydrogen bell-type furnace imported by Anshan Iron and Steel Cold Rolling Sheet Plant are all purchased from abroad. In 1991, the first phase of the all-hydrogen bell furnace was put into operation, and the value of the loss of computer components and burner controllers due to the high outer surface temperature of the heating cover amounted to DM 65000 within one year after it was put into operation. In 1995, the second phase of the all-hydrogen bell furnace, which was put into operation after improvement and perfection, was put into operation within one year. As the outer surface temperature of the heating cover maintained the design temperature, the loss value of computer components and burning controller was greatly reduced, only 8000 deutsche marks, I .e. saving 57000 deutsche marks, or 284000 yuan.

4. Conclusions
The following conclusions can be drawn from the production practice of the use of lightweight refractory brick lining in the early stage of the introduction of the bell furnace in Anshan Iron and Steel Cold Rolling Sheet Plant for several years and the introduction of a well-developed all-hydrogen bell furnace for one year using polycrystalline mullite fiber composite products vacuum-formed block lining and imported refractory lining:

(1) The vacuum forming block of domestic polycrystalline mullite fiber composite products meets the requirements of refractory materials in the heating and burning area of the all-hydrogen bell furnace, and can completely replace imported refractory materials.

(2) for all hydrogen bell type furnace heating cover burner area, compared with light refractory brick, can reduce the furnace repair time, improve equipment operation time, improve production capacity, save energy, reduce the consumption of imported spare parts.

(3) The use of polycrystalline mullite fiber composite products vacuum forming block to do all-hydrogen bell furnace burner area lining, significant economic benefits.